This AREA project aims to determine the function of surface adhesins in Veillonella's role as keystone species in periodontal dysbiosis. Periodontal diseases especially periodontitis affect the majority of people 65 years and older, and is the major cause of tooth loss in this population. Development of periodontitis involves a serial transition from healthy gum to gingivitis to periodontitis. It has been well established that periodontitis results from dysbiotic interactions between the subgingival microbiota and the host. In the center of this dysbiotic transition are Fusobacterium sp, especially F. nucleatum (Fn) and Veillonella sp (Va). Fn is a middle colonizer and ubiquitously present in all human dental biofilms. However, in healthy subjects, Fn exists in low abundance, while in subjects with gingivitis, its numbers increase substantially leading to gingival inflammation, which then causes increased exudates in the gingival pocket, enhancing growth of proteolytic, tissue destructive periodontopathogens. Fn also recruits many periodontopathogens to the biofilm via cell-cell coaggregation. While Fn is ubiquitously present in healthy biofilm, it cannot grow in vitro under conditions mimicking oral health, unless Va is present. Va is considered an early colonizer and ubiquitously present and highly abundant in oral biofilms. Va also coaggregates with Fn. These findings led us to hypothesize that it is the Va bacteria in the early biofilm that enable the growh of Fn, and by doing so function as keystone species in the subsequent dysbiotic transition of the oral biofilm. Two specific Aims are proposed to test this hypothesis. Aim 1 is to identify the functional domains of a streptococcal specific and an Fn specific adhesion, Aim2 is to determine the function of these adhesins on Fn growth in a 3- species biofilm model. We anticipate that by understanding the mechanisms of coaggregation and its effect on the growth of Fn, we could gain insights into the dysbiotic transformation of the dental biofilm by Veillonella. Currently, almost nothing is known about the earliest events leading to dysbiosis in the oral biofilm. Our study will have significant impact on development of strategies for periodontal disease prevention.